The present invention relates to a catalytic conversion process for xylenes containing ethylbenzene.
Among the xylenes, it is para-xylene that is in the greatest industrial demand at present, so the isomerization technique for converting ortho- and meta-xylenes which are in less demand into para-xylene is industrially important.
In general, industrially utilized xylenes are obtained by aromatic extraction and fractional distillation of reaction products from reforming or cracking of naphthas. But all of the crude xylenes thus obtained contain ethylbenzenes in addition to ortho-, meta- and para-xylenes. Therefore, ethylbenzenes is removed from such crude xylenes by some suitable means, and para-xylene is produced by the combination of separation step and isomerization step.
Ethylbenzene may be removed by direct separation thereof or by reaction thereof for conversion into a more useful compound. An example of the former is the distillation process wherein, however, it is necessary to perform an ultra-rectification because of a small difference in boiling point between ethylbenzene and xylenes, and this necessity results in increased equipment cost and running expenses. Thus, the distillation process is disadvantageous from the economic point of view. An example of the latter is a method wherein ethylbenzene is converted to xylenes by using a bifunctional catalyst comprising a platinum component and a solid acid component and at the same time there is performed isomerization of the xylenes. The method of removing ethylbenzene by the reaction is economically advantageous because it requires no special equipment. But a further improvement is desired because of problems involved therein, for example, platinum used in the above-mentioned bifunctional catalyst is a very expensive noble catalyst, and the conversion of ethylbenzene is restricted by a thermodynamic equilibrium relation between ethylbenzene and xylene isomers.
As the method for overcoming the above-mentioned problems, an increasing attention has recently been placed on a conversion process for ethylbenzene into benzene and ethane by using a catalyst comprising a solid acid component and a hydrogenation component. The hydrodealkylation reaction of ethylbenzene into benzene and ethane has features such that the improvement of conversion is relatively easy because the reaction scarcely undergoes thermodynamic restrictions, and that the resulting benzene is easily separable by the distillation process because of a large difference in boiling point from xylenes and has a high added value as a raw material for synthetic fibers and synthetic resins. As a catalyst for allowing such reactions to proceed efficiently, the present inventors have proposed a mordenite catalyst containing rhenium and/or phosphorus or this mordenite catalyst further containing molybdenum, tungsten and/or vanadium, previously in Japanese Laid Open Patent Publication Nos. 9727/1982, 64623/1982 and 134423/1982. But, to make this reaction more efficient, it is desirable to further improve the catalyst activity and selectivity.